In general, commercial gold electroplating processes utilize an aqueous bath wherein the metallic gold is present in the plus one oxidation state. Various alloying metals have been suggested including cobalt, nickel, molybdenum, rhenium, ruthenium, and tungsten. See Krienke et al. U.S. Pat. No. 3,910,774 and E. W. Turns, Plating and Surface Finishing, 64 (5) 46 (1977). In one system, where the cobalt and nickel are used in the form of their citrates, one can encounter hard but brittle gold alloy deposits which have an internal tensile stress with a tendency to form microcracks. It has also been found that upon deposition of the gold alloy from the cyanide/citrate system codeposited carbon may reach up to 0.2 or higher. This may cause poor thermal stability above 150.degree. C. with the formulation of undesirable deposit ruptures and/or blisters. Moreover, wear resistance which is rated as survival through 2000 passes on a cross wire tester for deposits of 2.5 .mu.m (100 .mu.inch) in thickness, is not as good as it might be for various commercial applications.
The use of gold deposited from the plus three oxidation state also has been suggested previously. Thus, Mohrnheim in Plating, 48 (10) 1104 (1961) described in the use of the tetracyanoaurate (III) ion complex in an electrodeposition system. The complex was formed in situ from the stoichiometric neutralization of the chloroaurate (III) acid with potassium hydroxide and the addition of potassium cyanide. A mixture of three salts was used to buffer the solution. At 60.degree. C., within the range of 0.2 and 2.0 ASD the deposits were bright, adherent, and showed fine crystallinity. The structure was attributed to the chloride ions present.
Knoedler, et al. Plating, 53 (6) 765 (1966) and Knoedler in Metalleberflache, 33 (7) 269 (1979) studied the characteristics of the cyanoaurate (III) complex in electrodeposition solutions. Several conclusions were developed showing: (a) that the rate at which the gold (III) complex tends to transform to the gold (I) complex increases with increasing pH and temperature; (b) that in the presence of metallic gold the cyanoaurate (III) complex and free cyanide will yield the cyanoaurate (I), thus discouraging the use of gold anodes; and (c) that at low potentials the cathodic reduction to monovalent gold occurs preferentially in acid solutions and those containing no free cyanide because of the shift in the redox potential to more positive values.
Freedman et al. in U.S. Pat. No. 3,598,706 claimed an electrodeposition solution for producing gold and gold based alloy deposits employing the cyanoaurate (III) complex. The preferred form was the cyanoauric acid as opposed to the potassium or sodium salt because of the advantage of not producing alkali metal hydroxide during the operation of the bath. The solution was operated at pH less than 3 and preferentially at 1.5 using citric and phosphoric acid, at current densities between 2 and 8.6 ASD and at temperatures between 35.degree. and 50.degree. C. The solution was buffered with a compound chosen from a group consisting of citrates, phosphates and tartrates. However, the use of the cyanoaurate (III) salts always contained a variable amount of the gold (I) complex and this was true no matter what method they used for producing the gold (III) salt. The cyanoaurate (I) complex decomposes precipitating AuCN at the low pH required for optimum deposition conditions.
Fletcher and Moriarty in the more recent U.S. Pat. No. 4,168,214 produced aqueous low pH gold and gold alloy deposition solutions containing the cyanoaurate (III) complex which was formed in situ. An excess of less than 15% above the stoichiometric amount of alkali metal cyanide was added to an auric chloride salt or acid to prevent the formation of the gold (I) oxidation state and the subsequent precipitation of aurocyanide. Hydrochloric acid was used to acidify the matrix and a water soluble metal chloride salt supplied the alloying metal. Ethylenediamine and alkali metal nitrate were also added to enhance conductivity. The former compound readily reacted with the acid to form ethylenediamine hydrochloride which provides a ready supply of chloride ions. The invention was aimed at electrodepositing gold on unactivated stainless steel, and it was felt that the presence of chloride and nitrate ions in an acid media provided a dilute aqua regia-like composition which would adequately prepare such a surface. Alloying metals such as nickel, cobalt, copper, tin, and indium were disclosed in this patent.
There are several drawbacks to the deposition systems described. The preparation of the plating solution is fairly involved requiring several reaction steps. Moreover, the presence of chloride ions could result in the evolution of chloride gas at the anode and become an environmental problem. If pulse plating is to be considered, the instability of the added organic compounds must be taken into consideration. Moreover, wear resistance of the electrodeposits is not as great as desired for many commercial applications.
From a review of the background literature certain criteria arise for any proposed method employing the gold (III) oxidation state. To have the cyanoaurate (III) complex remain stable, an acidic solution is required; however, at pH below about 3 gold (I) complex will precipitate as the aurocyanide and, therefore, the solution must be entirely free of the gold (I) oxidation state. In addition a nonchloride containing solution is obviously quite desirable.
One object of the present invention is to provide an improved gold (III) ternary alloy electroplating bath.
Another object of the present invention is to provide a trivalent gold ternary alloy electroplating bath which avoids the disadvantages attendant upon previously suggested gold (III) electroplating systems.
A further object of the present invention is to provide a gold (III), cobalt, and molybdenum ternary alloy electroplating bath which has markedly improved wear resistance.
These and other objects will become readily apparent from the ensuing description of the invention.